I need to buy a large cake tin approximately 18x8 inches and possibly at a reasonable price as I have to buy 2/3 to layer the cake. Where could I find such a large tin?

Thanks

Check online. 

Just wondering. ..will it fit in your oven? 

Globalsugarart.com has large cake pans. But do you mean 8" in height? A cake that tall would take for ever to bake. The edges, bottom and top would burn before the center is even cooked. 

They mean a rectangular tin 18" long and 8" wide I suspect. Check that it will fit in your oven before you buy!

when I make really large cakes (eg over 20" long) I just arrange two or more side by side eg here you could bake two 9" squares for each "layer" and then just trim a little off the long side to reduce the width.

Definately check your oven.  Some pans have a 'lip' on the ends - be sure you include that.  I found out the hard way.  The pans was noted as 16x12 BUT........with the extra lip on each side it was 18" long so it did not fit in my oven.

With large pans, unless they're something you're going to use very often, you're usually better off just buying one, since you're only going to be able to put one in your oven at a time in any case, and compared to the time needed to bake a cake that size, the time it takes to quickly wash it before reusing it is trivial.

Wilton makes a 12x18 pan that you may be able to get at a good price with a coupon from Michaels or JoAnn.

The suggestion to use 2 9x9 pans side by side is also a good one.  You can trim the 9" to 8" if needed, and they'll bake a lot faster than a larger pan would.

Here's an example where I made a 14x11 3 layer cake using six 7x11 cakes.

(In this case I didn't worry about getting filling all the way to the edges, because I was going to carve it down into something else.  The cardboard sticking up in the middle was a template for carving.)

I actually rent the really big tins that I only use very occasionally - my local cake decorating store offers this service. But you do only need one as explained above.